The invention relates to a method for checking the dynamic behavior of a measuring sensor which detects at least one substance concentration in the exhaust tract of an internal combustion engine operating with excess air, downstream of an NOx storage catalytic converter which catalytically converts stored NOx by the addition of a reducing agent, in a regeneration phase. The reducing agent is produced by operating the internal combustion engine briefly with a rich air/fuel mixture (lambda less than 1).
Internal combustion engines with lean combustion are employed with increasing frequency to reduce the fuel consumption of spark ignition internal combustion engines even further. It is necessary to have special exhaust gas retreatment in such internal combustion engines in order to meet the required exhaust gas emission limit values. NOx storage catalytic converters are used for that purpose. Those Nox storage catalytic converters, by virtue of their coating, are capable of absorbing NOx compounds from the exhaust gas which occur in a storage phase during lean combustion. The absorbed or stored NOx compounds are converted during a regeneration phase, by the addition of a reducing agent, into harmless compounds. CO, H2 and HC (hydrocarbons) may be used as reducing agents for lean-operated spark ignition internal combustion engines. Those agents are produced by briefly operating the internal combustion engine with a rich mixture and are made available to the NOx storage catalytic converter as exhaust gas components. As a result, the stored NOx compounds in the catalytic converter are broken down.
A measuring sensor is provided downstream of the NOx storage catalytic converter in the exhaust tract for detecting at least one substance concentration, in order to control the regeneration and storage phases or to check the NOx storage catalytic converter. Measuring sensors which emit an NOx signal representing the NOx concentration are normally used. Such a measuring sensor is known, for example, from a publication entitled xe2x80x9cPerformance of Thick Film Nox Sensor on Diesel and Gasoline Enginesxe2x80x9d, by N. Kato, Y. Hamada and H. Kurachi, in Society of Automotive Engineers, Publication No. 970858.
Self-diagnosis (On Board Diagnosis=OBD) of the entire exhaust gas retreatment system is increasingly demanded to ensure that the required exhaust gas emission limit values are adhered to over the entire service life of such an internal combustion engine. Such OBD is necessary, in particular, for the dynamic behavior of a measuring sensor that is used. In that case, a slowdown in the response of the measuring sensor should be able to be detected and, if there is an inadequate dynamic behavior, a faulty measuring sensor should be able to be diagnosed.
In contrast to a lambda probe disposed upstream of a catalytic converter, the exhaust gas downstream of a catalytic converter, in particular an NOx storage catalytic converter, normally does not have any pronounced substance concentration fluctuations. Therefore, it is difficult to recognize a slowed-down response of a measuring sensor.
It is accordingly an object of the invention to provide a method for checking the dynamic behavior of a measuring sensor disposed downstream of an NOx storage catalytic converter in the exhaust tract of an internal combustion engine for detecting a substance concentration, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for checking the dynamic behavior of a measuring sensor detecting at least one substance concentration, which comprises providing an internal combustion engine operating with excess air and producing a reducing agent by operating briefly with a rich air/fuel mixture. An exhaust tract is provided for the internal combustion engine. An NOx storage catalytic converter is placed in the exhaust tract for catalytically converting stored Nox by an addition of the reducing agent in a regeneration phase. The measuring sensor is placed downstream of the NOx storage catalytic converter in the exhaust tract for emitting a measuring sensor signal representing a substance concentration. A rate of change of at least one substance-concentration is monitored during the regeneration phase. A faulty dynamic behavior of the measuring sensor signal is diagnosed when the rate of change is not reproduced with sufficient edge steepness by the measuring sensor signal.
The invention proceeds from the knowledge that, in the regeneration phase of the NOx storage catalytic converter, substance concentration changes occur with predetermined rates of change. Thus, the dynamics of the measuring sensor can be checked by monitoring the rate of change of a substance concentration. It is thereby possible to check the dynamics of a plurality of signals from a measuring sensor which represent different substance concentrations, depending on the embodiment of the method according to the invention. An NOx signal representing the NOx concentration, a lambda signal representing the lambda value and an O2 signal representing the oxygen concentration, are particularly relevant.
It is therefore important that the rate of change of one or more substance concentrations in the regeneration phase be monitored, since the changes in the substance concentration are then caused by reactions in the NOx storage catalytic converter itself or changes produced in the lambda value appear with sufficient amplitude at the measuring sensor. Such dynamics cannot be produced at any other operating point, since, in a lambda≈1 operating mode, the fluctuations in the substance concentration or in the lambda value downstream of the catalytic converter are too low or are damped by the catalytic converter. In normal operation, this damping makes it impossible to produce substance concentration changes with a high rate of change in order to check the measuring sensor dynamics through the use of mixture changes while the internal combustion engine is in operation.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for checking the dynamic behavior of a measuring sensor in the exhaust tract of an internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.